Source Editor: allow extensions to be un-used as well as used

At the moment, we can use as many extensions as possible. However, if the environment changes and we do not need one or more extensions there's currently no way to "un-use" those so that they would not provide unnecessary or conflicting functionality.

I have created some snippets with different potential patterns we could employ for extensions in the Source Editor and I would like to use this issue as a point pushing that effort forward.